Up to now, attempts have been made to elevate the recording density of, for example, an optical disc, used as a recording medium for a disc-shaped recording medium for the audio information or the picture information, by a recording/reproducing apparatus used for recording data processed by the computer system. For elevating the density of this sort of the recording medium, the recording track formed on the recording medium for recording information signals is reduced in pitch, and the recording pit is also reduced in size, for elevating the recording density per unit area.
For recording and/or reproducing the information signals for an optical disc improved in the recording density, it is required to use an optical pickup device capable of reducing the spot size of the light beam illuminated on the optical disc.
The optical pickup device for illuminating a converged light beam on an optical disc employs a semiconductor laser element as a light source and shapes the light beam radiated by this semiconductor laser element by an optical component to split the light beam into plural light beam portions which are condensed on the optical disc. This optical pickup device includes a photodetector for detecting a return light beam reflected from the optical disc and detects the return light beam by this photodetector to detect the information signals recorded on the optical disc in order to reproduce the information. The optical pickup causes the light beam radiated from the semiconductor laser element to be converged and illuminated as fine-sized beam spot on the optical disc.
The beam spot diameter of the light beam illuminated on an optical disc is given by the wavelength .lambda. of the light beam and the numerical aperture NA of an objective lens of the optical pickup device configured for condensing the light beam on the optical disc. Specifically, the beam spot diameter of the light beam illuminated on the optical disc is given by .lambda./NA. Therefore, if the wavelength .lambda. of the light beam used is constant, it is necessary to increase the numerical aperture NA of the objective lens used for converging the light beam.
Meanwhile, in a light beam radiated from a semiconductor laser element used as a light source of the optical pickup device, there is produced a change in the radiation angle between the TE direction (direction of the electrical field) and the TM direction (direction of the magnetic field). That is, the radiation angle of the light beam radiated from the semiconductor laser element is not uniform when seen from the light-emitting point, and differs from one light beam radiated from each light emitting point to another. If the light beams having different radiation angles are converged by the objective lens to form a beam spot on the optical disc, the beam spot formed on the optical disc becomes elliptically-shaped or is not converged to a desired size but is increased in spot diameter because the radiation angle of the light beam radiated from each light-emitting point is not constant.
For overcoming these inconveniences, there is mounted a beam shaping element on the light path of the light beam radiated from the semiconductor laser element for enlarging or narrowing the light beam radiated from the semiconductor laser element in one direction.
As the beam shaping element, a lens termed an anamorphic lens is usually employed. If this anamorphic lens is placed on a divergent light path of the light beam radiated from the semiconductor laser element, there is produced aberration, such as astigmatic aberration or coma aberration. It is therefore necessary to provide the anamorphic lens in the light path of the light beam collimated by, for example, a collimator lens. That is, in the optical pickup device employing the anamorphic lens as the beam shaping element used for shaping the light beam, it is necessary to provide a collimator lens for collimating the light beam radiated as a divergent light beam from the semiconductor laser element. Thus, not only can the optical path length from the semiconductor laser element to the anamorphic lens not be reduced, but also it is difficult to reduce the size of the device.
For shaping the shape of the beam spot formed on the optical disc, there is proposed an optical pickup device in which a cylindrical lens is arranged on the optical path of the light beam radiated from the semiconductor laser element and in which a plan-parallel glass plate is arranged at a pre-set angle relative to the optical axis of the light beam. With this optical pickup device, it is difficult to remove completely the aberration in the beam spot of the light spot converged on the optical disc.